"Mitt Romney, the presumed Republican nominee for president, promises to lower everyone’s income tax rate without reducing revenue. This sounds terrific. Why didn’t we think of it sooner?"

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If these are “garbage assumptions,” why doesn’t Mr. Romney let us in on his own? If he can be specific about how much he would lower the tax rate, why not be as specific about how he would pay for that? A spokeswoman responded: “Governor Romney has announced clear and specific principles for his tax reform plan and will work with Congress to write a bill that achieves them.”

In reality, his principles are mutually exclusive: You can’t simultaneously lower tax rates, take in as much money as before and protect the middle class. There may be no politically feasible way, and there’s certainly no politically popular way, to “broaden the base” enough to pay for Mr. Romney’s tax cuts. It’s reasonable to assume that his cuts would, as did President Bush’s, worsen the nation’s deficit.

Until he’s willing to explain how he would avoid such a result, he has little standing to criticize Mr. Obama’s fiscal shortcomings.